LADERA RANCH, Calif.—Rising home prices have fueled the return of a practice that some blamed for inflating the bubble: house flipping.

In California, the number of homes sold in recent months that had been flipped—or bought and resold within six months—has reached the highest levels since late 2005, according to PropertyRadar, a real-estate data firm. About 6,000 homes have been flipped in the state this year through April, or more...